The events that regulate the development and proliferation of hematopoietic cells are precise, complicated, flexible, and subject to many levels of controls. The control mechanisms involve both humoral and cell-associated factors from the microenvironement as well as pre-programmed patterns of gene expression intrinsic to different progenitor and stem cells. There have been two basic goals of this project over the last 18 years. The first goal has been to understand the external signals that influence patterns of differentiation, control traffic of mature and immature cells, and influence functions of mature hematopoietic cells. The second goal has been to understand how leukemia oncogenes disrupt, conscript, or deregulate the normal mechanisms that control hematopoiesis. Over the initial five years of this grant (1984-88), our aims were to identify the major cytokines affecting myelopoiesis and determine their major biological properties first in vitro, and then in vivo in human subjects. Over the next five years of the grant (1989-93), our aims were to identify the receptors for the major myeloid growth factors, begin to understand how these cytokines interact physically with their receptors, and identify the initial elements of receptor signaling. Studies during this period also led to the discovery of the significant degree of overlap between the actions of activated tyrosine kinase oncogenes, such as BCR/ABL or TEL/ABL, with the actions of ligand-activated cytokine receptors. The similarities, and differences, between BCR/ABL and IL-3 receptor function, for example, explain many of the clinical phenotypes of CML. Over the last grant period (1994-2002) the goals were to investigate in detail how signals travel from cytokine receptors to the nucleus, how positive and negative signals are integrated in hematopoietic cells, and how these different signals are similar, or distinct, from those initiated by leukemia oncogenes such as BCR/ABL. Overall, the factors, receptors, and signals that regulate proliferation and viability of many lineages of hematopoietic cells are now fairly well understood. The signals of leukemia and myeloproliferative syndrome oncogenes that induce proliferation and viability are also becoming much more clear. The external signals that regulate differentiation and lineage commitment of immature hematopoietic cells, however, are still poorly defined. There is increasing evidence that receptors of the Notch family participate in some differentiation "decisions" of stem cells, such as commitment to the lymphoid vs. myeloid lineages at an early time point. In many tissues, and in animals from flies to humans, Notch receptors determine certain types of lineage commitment, typically forcing a stem cell to commit to one lineage at the expense of another lineage. It is proposed here to determine how the signals from Notch receptors mediate these effects, using hematopoietic cells as the model system. The goals of the proposed studies will be to determine how Notch signaling influences differentiation and lineage commitment in hematopoietic cells, better define the functions of Notch in hematopoiesis, and understand the mechanism of transformation of stem cells by the human ALL oncogene, TAN-1 (a mutant allele of the Notch 1 receptor).